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A Three-State Model for the Photo-Fries Rearrangement Josene Toldo, Mario Barbatti, Paulo F

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A Three-State Model for the Photo-Fries Rearrangement Josene Toldo, Mario Barbatti, Paulo F A three-state model for the Photo-Fries rearrangement Josene Toldo, Mario Barbatti, Paulo F. B. Gonçalves To cite this version: Josene Toldo, Mario Barbatti, Paulo F. B. Gonçalves. A three-state model for the Photo-Fries rearrangement. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2017, 29 (19), pp.19103-19108. 10.1039/C7CP03777E. hal-02288764 HAL Id: hal-02288764 https://hal-amu.archives-ouvertes.fr/hal-02288764 Submitted on 16 Sep 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Please do not adjust margins A three-state model for the Photo-Fries rearrangement Josene M. Toldo,a,b Mario Barbattib and Paulo F. B. Gonçalvesa A three-state model for the Photo-Fries rearrangement (PFR) is proposed based on multiconfigurational calculations. It provides a comprehensive mechanistic picture of all steps of the reaction, from the photoabsorption to the final tautomerization. The three states participating in the PFR are an aromatic 1ππ*, which absorbs the radiation; a pre- dissociative 1nπ*, which transfers the energy to the dissociative region; and a 1πσ*, along which dissociation occurs. The transfer from 1ππ* to 1nπ* involves pyramidalization of the carbonyl carbon, while transfer from 1nπ* to 1πσ* takes place through CO stretching. Different products are available after a conical intersection with the ground state. Among them, a recombined radical intermediate, which can yield ortho-PFR products after an intramolecular 1,3-H tunneling. The three- state model is developed for phenyl acetate, the basic prototype for PFR, and it reconciles theory with a series of observations from time-resolved spectroscopy. It also delivers a rational way to optimize PFR yields, since, as shown for four different systems, diverse substituents can change the energetic order of the 1ππ* and 1nπ* states, preventing or enhancing PFR. phenol. In addition, the reaction quantum yield of rearranged Introduction products is strongly influenced by solvent polarity as well as by the presence of electron donor or acceptor in the aromatic moiety.19, 27, 32, 44, 45 Photo-Fries rearrangement (PFR)—a photochemical conversion of aryl esters to ortho- and para-hydroxyphenones (Scheme 1)—is a key step in the synthesis of a large number of compounds.1-4 It also plays an important role in the design of functional polymers5-8 and in the photodegradation of drugs9, 10 and agrochemicals.11-13 Compared to its thermal version, the Lewis-acid catalyzed Fries rearrangement, PFR has an additional benefit of being a greener synthetic route, since it can be achieved under milder conditions.3, 14, 15 Given its importance for synthesis, it is not surprising that PFR has been the subject of numerous investigations in the past.15-36 Nevertheless, the conceptual theoretical knowledge of this reaction is still incipient36- 38 and, as we shall see, even the full set of electronic states involved in the reaction has not been yet identified. Experimental observations have established that PFR takes place 36, 37 in the lowest singlet state (S1) although, in some cases, a contribution from upper triplet states is also expected.34, 39-41 The homolytic cleavage of the OC–O bond gives rise to a carbonyl and phenoxyl radical pair. The subsequent recombination leads to the starting ester and to cyclohexadienone intermediate, which Scheme 1. General scheme of Photo-Fries rearrangement. For phenyl tautomerizes to yield the rearranged products. The final step is a acetate, R = methyl. hydrogen shift, which can proceed either via tunneling or through Further insights into the early events in PFR of phenyl acetate solvent rearrangement.42, 43 Alternatively, the radicals can escape (PA) in cyclohexane has been provided using transient electronic and from the solvent cage leading to formation of the corresponding vibrational absorption spectroscopies. Pumping at 267 nm, they show radical pairs being formed within 28 ps, although phenoxyl a. Department of Physical Chemistry, Federal University of Rio Grande do Sul, Av. radicals are observed as early as 15 ps.36 Two-color femtosecond Bento Gonçalves, 9500, Porto Alegre-RS, CEP 91501-970, Brazil. pump-probe spectroscopy pumped at 258 nm, revealed that the S1 b. Aix Marseille Univ, CNRS, ICR, Marseille, France. 1 *Corresponding authors : [email protected] (JMT), mario.barbatti@univ- state of para-tBu-PA, also in cyclohexane, is depopulated via * amu.fr (MB), [email protected] (PFBG). within just 2 ps and the dissociated radicals recombine within 13 ps.37 Electronic Supplementary Information (ESI) available: Molecular orbitals, absorption spectra, reaction path, energies, and Cartesian coordinates. See In contrast to a large number of experimental studies, the last DOI: 10.1039/x0xx00000x theoretical investigation on PFR was delivered by Grimme, in 1992, Please do not adjust margins Please do not adjust margins using semi-empirical methods.38 In that work, barriers between 0.9 was used and an imaginary level shift48 of 0.1 a.u was applied to deal and 1.2 eV were found for PA photodissociation starting from a 1nπ with intruder states. The ANO-S-VDZP49 basis set was employed in all * state. Such large barriers are clearly incompatible with the calculations. The S1/S0 conical intersection was initially optimized at measured picosecond time scale of the process.36, 37 Moreover, still the CASSCF level. Due to the usual energy split when CASPT2 is due to methodological limitations of that early work,38 the relative computed for such geometries,50 the intersection was further importance of dissociation along 1nπ* versus 1πσ* could not be relaxed at CASPT2 level. Thus, starting from the CASSCF geometry, clearly stated. In fact, the lack of high-level theoretical information restricted optimizations along the CO–O bond (R) were done at the on PFR is such that even the character of the initial excited state— MS3-CASPT2(6,6) level, until the S1 and S0 states became 1nπ* or 1ππ*—has still been under debate.36-38 degenerated. For this final intersection geometry, energies were Given the knowledge gap between theory38 and the most recent computed at MS3-CASPT2(14,12). The subsequent pathway after the experimental works,36, 37 our aim has been to provide a CI was optimized in the ground state at the CASSCF level, still along comprehensive picture of PFR, based on high-level constrained values of R. The branch yielding the radical pair was multiconfigurational theoretical methods, applied to PA in the calculated starting from large values of R, while the branch giving rise gas phase, the minimum prototype to understand PFR. to PFR was calculated systematically increasing R starting from the CI The multiconfigurational theoretical approach has allowed us to structure. All calculations were carried out using MOLCAS 8 clarify several the following questions: Which state is initially program.51 populated? How is the energy transferred from the Franck-Condon region to the dissociative pathway? What are the electronic states involved and their multiplicities during dissociation? Is there any Results and discussions relevant conical intersection along the way? Why does the The PFR mechanism photoexcited population branches into dissociated and The analysis of relaxed reaction pathways in the excited states rearranged species? How does tautomerization occur? computed with MS-CASPT2//CASSCF shows that after Our results revealed that PFR involves three electronic excited photoexcitation, PFR takes place through the S state involving three states arranged along a specific topography that allows transferring 1 diabatic characters. A schematic potential energy profile the photoenergy from the aromatic to the carbonyl region. To summarizing this three-state model is shown in Figure 1 for PA. Along further explore this three-state model for PFR, we extended the the solid lines, the OC-O bond distance is the main reaction calculations for three other aromatic esters containing an coordinate, while along the dashed curve, the hydrogen shift amino group instead of a methyl group attached to carbonyl between the oxygen and ortho carbon is the main reaction moiety. As result, we succeed in providing a solid conceptual basis coordinate. Although the relative energies in this figure correspond for a class of reactions important for organic, polymer, and to those for PA, this three-sates profile is still valid for other environmental chemistry, answering questions that have hindered molecules undergoing PFR, as discussed later. progress in these fields and laying the groundwork for interpreting four decades of experiments. Computational details Theoretical calculations were carried out using MS-CASPT2//CASSCF protocol,46 in which energies are computed at the multi-state complete active space second-order perturbation theory (MS-CASPT2) on structures optimized at the complete active space self-consistent field level (CASSCF). Critical points (minima, transition states, and conical intersection) and reaction paths were optimized with an active space including 14 electrons in 12 orbitals and state-averaged over three states (SA3-CASSCF(14,12)). Cartesian coordinates for all these structures are given in the Supporting Information (SI). The active space for PA was composed of seven occupied and five virtual orbitals: 4π and 4 π *, 1 orbital pair σ/σ* along the OC–O bond, and two non-bonded Figure 1. Schematic overview of the three-state model for PFR electrons pairs, one in the oxygen of the carbonyl group and applied to PA. The insets show the main orbital transitions of the another in the oxygen bonded to the phenyl ring (see SI1).
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